200 merc,,,what did i do wrong???

[hillbillyfitter]

its a 1989 200hp merc,just got it,ran fine first to or three times.It seemed it was losing a little power.First time i fill the tank i use the 50 to 1 ratio on the bottle.Thought something didnt feel as smooth,took it out the following day and would not break plane,not even enough power to get it back on the trailer,also it started to sound real rough with throttle engaged bot no gas applied.Took out plugs,they look fouled,but why the sudden change???oil to gas ratio?plugs?Any help?Thanks